1. Field the Invention
The present invention relates to an auto focusing apparatus and method. More specifically, the present invention is directed to an auto focusing apparatus and method for automatically adjusting a focus when linewidth of a pattern formed on a semiconductor wafer is measured.
2. Description of Related Art
Most semiconductor devices may be manufactured by forming desired patterns, which may be formed by iteratively performing multiple processes such as formation of various thin films on a wafer surface, exposure using a given mask, and selective etching.
A critical dimension (CD) and a profile of a pattern copied on a wafer surface may vary with focus magnitude of an exposure system.
The shorter an exposure wavelength may be, the greater the variation of the CD and/or the profile. Therefore, monitoring of the focusing process may be significant in a semiconductor manufacturing industry. As a result, a typical exposing system used in semiconductor manufacturing may have an auto focusing apparatus to achieve improved or optimal focusing.
If a thin film is selectively etched to form a pattern on a wafer surface, a worker may start to check whether a pattern forming process is being performed correctly according to a given design. The worker may check whether the pattern formed on the wafer surface is correctly aligned with a previously formed circuit pattern and/or whether linewidth of a pattern copied on a wafer surface is of a desired size.
The most general method for measuring optimal focusing may be after a specific-shaped pattern is formed on a wafer, variation of a critical dimension and/or a profile of the pattern may be checked by alternating the focus. However, the variation may be determined by a worker's naked eye using a scanning electron microscope (SEM). As a result, the reliability of the determination may be dependent up differences between individuals and/or skill of the worker.
Another auto focusing method has been proposed, where a magnitude of a defocus may be computed using a phase shift mask and a pattern is shifted along a focus.
Apart from the above, there are other proposals, in which auto focusing of an objective lens is described with relation to a measurement of a pattern linewidth.
As integration density of semiconductor devices increases and/or a tolerance is reduced, a linewidth measuring value may be varied according to the skill of a worker and/or a reliability for auto measurement. Thus, auto focusing and linewidth measurement may be the current trend for improving reliability and/or accuracy of alignment.
In recent years, there has been an increasing requirement for monitoring defects to determine whether the defects may be a pattern profile linewidth size abnormality or a linewidth size abnormality caused by poor auto focusing. Particularly, defects resulting from poor auto focusing may increase resulting in interlock, occurrences of abnormalities, etc.
However, a conventional linewidth measuring apparatus may not have a method or system for detecting erroneous auto focusing and/or problems caused by erroneous auto focusing. Thus, the conventional linewidth measuring apparatus may mistakenly measure a pattern having a desire-sized linewidth as an erroneous pattern due to a poor focusing. In particular, mistakenly measuring a normal pattern as an erroneous pattern may have an effect on a photolithographic process which may lead to yield reductions. Further, when interlock and/or other problems occur because erroneous data is measured by poor focusing, additional time may be required for analyzing and/or checking a measured value which may result in production delays.